When the Brompton is in folded position, it is indeed much easier to carry with the handle. This is especially so if you’ve replaced the original saddle with an aftermarket one. It’s such an improvement that it outweighs the two non-negligible negative points.

First, when the Brompton is extended, it is slightly more difficult to carry. This is definitely the case if you’ve got front-loaded luggage, such as a C Bag. It might not be the case if you’ve got no luggage as well as the stock saddle, but I can’t easily test this.

Second, the Off Yer Bike seriously interferes with the fold. If the rear wheel is properly inflated, it doesn’t swing forward as far as it should (a half cm of difference can really matter sometimes), which makes it difficult to push the seat tube down; if the tube isn’t all the way down, the fold isn’t secure and the bike will come apart when you lift it. This is a not inconsiderable design flaw.

The best workaround I’ve found so far is this:

Partially fold the bike: the rear wheel should be in and the front wheel should be back, as in this photo.

Do something to keep the bike from rolling backward or forward. I do this by stepping on one of the rollerwheels.

Loosen the seat clamp and hold the saddle with one hand.

With your other hand, push the handlebar stem forward and down. It doesn’t need to move much, just enough to get the fold as compact as it would be without an Off Yer Bike on the cross bar.

While holding the handlebar stem forward and down, lower the saddle all the way.

Let go of the handlebar stem, tighten the seat clamp, and fold the handlebars down.

Once you get the hang of this it isn’t much more difficult or slower than than folding an offyerbikeless Brompton.